Low-profile resins have been described that contain unsaturated polyester resins, thermoplastic low-profile additives, and a polymerizable monomer, usually styrene. In addition to these components other materials have been added to low-profile systems to improve specific properties.
The Iseler, et al. U.S. Pat. No. 4,622,354 describes "phase stabilizing agents" that comprise a select group of compounds from three classes: fatty acids, dimer acids and polyester polyols. When used in an SMC formulation where the thermoplastic low-profile additive is polymethylmethacrylate and a urethane prepolymer is included, the phase stabilizing agent reduces the gross separation that occurs during the maturation process. The resin compositions described by Iseler et al. are two-pack systems that formerly phase-separated during maturation prior to the addition of the phase stabilizers.
The Ochsenbein et al. U.S. Pat. No. 4,473,544 describes an anti-shrink additive with a tri- or tetrafunctional polyether condensation product of propylene oxide or a triol or tetrol wherein the condensation product is acidified in such a manner that it possesses at least one terminal acidic functional group per elementary molecule. This material is used as a low-profile additive.
The Atkins U.S. Pat. No. 4,555,534 describes low-shrink pigmentable unsaturated polyester resins which comprises a polyester resin comprising the reaction product of an olefinically unsaturated dicarboxylic acid or anhydride and a polyol, an olefinically unsaturated monomer, a thickening agent, a pigment, a carboxylated vinyl acetate polymer low-profile additive, and a surface active compound. The Atkins '534 patent describes low-shrink resins having improved uniformity of pigmentation in internally pigmented thickened polyester molding compositions. These pigmentable resin systems are low-shrink and not low-profile. The surface quality of these pigmentable systems is considerably inferior to surface required for automotive appearance applications.
Although the use of low-profile additives and thickening agents, as described, do effect some degree of improvement in the anti-shrinkage and surface smoothness characteristics of the unsaturated polyester compositions, it is still not possible to achieve the degree of surface smoothness required of today's thermoset molded articles.